1. Field of the Invention
This invention relates generally to input circuits and more specifically to a circuit for inputting analog voltage signals from a plurality of channels to a controller.
2. Description of the Prior Art
A micropowered microcontroller is available in the market which uses CMOS technology to reduce its power consumption to as little as five watts. The microcontroller is designated as the 8640 Industrial Microcontroller made and sold by Texas Instruments Incorporated and is an intelligent, programmable device which integrates measurements, computation, control and communications in a stand alone industrial package. The microcontroller provides an array of inputs, including, eight differential analog input channels multiplexed into a programmable gain amplifier which provides several input ranges. An autoranging alogorithm can be enabled for each channel to automatically select the appropriate input gain range to track a changing input signal. The analog ground voltage, the analog reference voltage and the reference voltage temperature are periodically measured to automatically compensate each field channel measurement for offset and span changes in the circuit.
However, when used in hostile environments which include a common mode voltage, some means of isolation or front end signal conditioning is required, as by using transmitters to perform the isolation, isolation amplifiers on each of the channels, or the like. That is, in many environments, such as a typical factory environment, it is very difficult to consistently follow effective grounding procedures so that the possibility of the existence of a common mode both initially as well as over the course of time is very high. In order to overcome this problem the user of a microcontroller has to provide front end signal conditioners of some sort as mentioned above. For example, transmitters can be used with each channel to perform the required isolation and then the input circuit need only provide transient protection. However, on a channel by channel basis this can be very expensive and represents an undesirable cost burden.
Typically isolation can be effectively obtained by using reed relays because of their high stand-off voltage and very low on resistance however they require too much power and are not fast enough for use with high speed, multiplexed channel inputs of the microcontroller. Similarly, a "flying capacitor" approach which inherently involves time for stabilization of pump out current is also too slow to be effective in such a system.
In order for the microcontroller to function as intended, the input circuit must have in addition to having input-output and channel to channel isolation, a high single channel digitizing rate, high resolution, gain switching, high accuracy, transient immunity, wide temperature range, low power, high reliability, low cost and at least medium multiplexing speed. With regard to software it must have signal integration, offset, gain and temperature drift compensation and autoranging.